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d="M19,2H5C3.3,2,2,3.3,2,5v14c0,1.7,1.3,3,3,3h14c1.7,0,3-1.3,3-3V5C22,3.3,20.7,2,19,2z M20,19c0,0.6-0.4,1-1,1H5c-0.6,0-1-0.4-1-1V5c0-0.6,0.4-1,1-1h14c0.6,0,1,0.4,1,1V19z"></path><path d="M16,11h-3V8c0-0.6-0.4-1-1-1s-1,0.4-1,1v3H8c-0.6,0-1,0.4-1,1s0.4,1,1,1h3v3c0,0.6,0.4,1,1,1s1-0.4,1-1v-3h3c0.6,0,1-0.4,1-1S16.6,11,16,11z"></path></svg></div></div><div class="items" data-v-6e45c352><!--[--><!--[--><a class="VPLink link link" href="/docs/frontend/js/JavaScript%E7%9A%84%E5%9F%BA%E7%A1%80%E8%AF%AD%E6%B3%95.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>基础语法</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/docs/frontend/js/%E5%88%86%E6%94%AF%E7%BB%93%E6%9E%84.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>分支结构</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/docs/frontend/js/%E5%BE%AA%E7%8E%AF%E7%BB%93%E6%9E%84.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>循环结构</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link active" href="/docs/frontend/js/%E5%87%BD%E6%95%B0(%E4%B8%8A).html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>函数（上）</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/docs/frontend/js/%E5%87%BD%E6%95%B0%EF%BC%88%E4%B8%8B%EF%BC%89.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>函数（下）</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/docs/frontend/js/%E6%95%B0%E7%BB%84.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>数组</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/docs/frontend/js/%E5%AD%97%E7%AC%A6%E4%B8%B2%E7%9A%84%E6%93%8D%E4%BD%9C.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>字符串的操作</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/docs/frontend/js/Math%E5%92%8CDate.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>Math和Date</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/docs/frontend/js/BOM%E5%92%8CDOM%EF%BC%88%E4%B8%8A%EF%BC%89.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>BOM和DOM（上）</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/docs/frontend/js/DOM%EF%BC%88%E4%B8%8B%EF%BC%89.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>DOM（下）</span><!--]--><!----></a><!----><!--]--><!--]--></div></section></div><div class="group" data-v-a186aa16><section class="VPSidebarGroup collapsible collapsed" data-v-a186aa16 data-v-6e45c352><div class="title" role="button" data-v-6e45c352><h2 class="title-text" data-v-6e45c352>千锋线下学习</h2><div class="action" data-v-6e45c352><svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" viewbox="0 0 24 24" class="icon minus" data-v-6e45c352><path d="M19,2H5C3.3,2,2,3.3,2,5v14c0,1.7,1.3,3,3,3h14c1.7,0,3-1.3,3-3V5C22,3.3,20.7,2,19,2zM20,19c0,0.6-0.4,1-1,1H5c-0.6,0-1-0.4-1-1V5c0-0.6,0.4-1,1-1h14c0.6,0,1,0.4,1,1V19z"></path><path d="M16,11H8c-0.6,0-1,0.4-1,1s0.4,1,1,1h8c0.6,0,1-0.4,1-1S16.6,11,16,11z"></path></svg><svg version="1.1" xmlns="http://www.w3.org/2000/svg" viewbox="0 0 24 24" class="icon plus" data-v-6e45c352><path d="M19,2H5C3.3,2,2,3.3,2,5v14c0,1.7,1.3,3,3,3h14c1.7,0,3-1.3,3-3V5C22,3.3,20.7,2,19,2z M20,19c0,0.6-0.4,1-1,1H5c-0.6,0-1-0.4-1-1V5c0-0.6,0.4-1,1-1h14c0.6,0,1,0.4,1,1V19z"></path><path d="M16,11h-3V8c0-0.6-0.4-1-1-1s-1,0.4-1,1v3H8c-0.6,0-1,0.4-1,1s0.4,1,1,1h3v3c0,0.6,0.4,1,1,1s1-0.4,1-1v-3h3c0.6,0,1-0.4,1-1S16.6,11,16,11z"></path></svg></div></div><div class="items" data-v-6e45c352><!--[--><!--[--><a class="VPLink link link" href="/docs/frontend/qf/JS%E8%BF%90%E7%AE%97%E7%AC%A6.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>JS运算符</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/docs/frontend/qf/JS%E5%BE%AA%E7%8E%AF%E5%AF%B9%E8%B1%A1.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>JS循环对象</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/docs/frontend/qf/JS%E5%87%BD%E6%95%B0.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>JS函数</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/docs/frontend/qf/JS%E5%AF%B9%E8%B1%A1.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>JS对象</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/docs/frontend/qf/JS%E6%95%B0%E7%BB%84%E9%87%8D%E6%9E%84.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>JS数组重构</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/docs/frontend/qf/JS%E6%95%B0%E7%BB%84%E9%87%8D%E6%9E%842.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>JS数组重构2</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/docs/frontend/qf/JS%E8%B7%AF%E7%94%B1.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>JS路由</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/docs/frontend/qf/JSDOM.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>JSDOM知识</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/docs/frontend/qf/JS%E4%BA%8B%E4%BB%B6.html" style="padding-left:0px;" tabindex="-1" data-v-70e2cdfb data-v-3c355974><!--[--><span class="link-text" data-v-70e2cdfb>JS事件</span><!--]--><!----></a><!----><!--]--><!--]--></div></section></div><!--]--><!--[--><!--]--></nav></aside><div class="VPContent has-sidebar" id="VPContent" data-v-5211c0b1 data-v-296cfad2><div class="VPDoc has-sidebar has-aside" data-v-296cfad2 data-v-cfb513e0><div class="container" data-v-cfb513e0><div class="aside" data-v-cfb513e0><div class="aside-curtain" data-v-cfb513e0></div><div class="aside-container" data-v-cfb513e0><div class="aside-content" data-v-cfb513e0><div class="VPDocAside" data-v-cfb513e0 data-v-afc4c1a1><!--[--><!--]--><!--[--><!--]--><div class="VPDocAsideOutline" data-v-afc4c1a1 data-v-2865c0b0><div class="content" data-v-2865c0b0><div class="outline-marker" data-v-2865c0b0></div><div class="outline-title" data-v-2865c0b0>On this page</div><nav aria-labelledby="doc-outline-aria-label" data-v-2865c0b0><span class="visually-hidden" id="doc-outline-aria-label" data-v-2865c0b0> Table of Contents for current page </span><ul class="root" data-v-2865c0b0 data-v-1188541a><!--[--><!--]--></ul></nav></div></div><!--[--><!--]--><div class="spacer" data-v-afc4c1a1></div><!--[--><!--]--><!----><!--[--><!--]--><!--[--><!--]--></div></div></div></div><div class="content" data-v-cfb513e0><div class="content-container" data-v-cfb513e0><!--[--><!--]--><main class="main" data-v-cfb513e0><div style="position:relative;" class="vp-doc _docs_frontend_js_%E5%87%BD%E6%95%B0(%E4%B8%8A)" data-v-cfb513e0><div><h1 id="函数-上" tabindex="-1">函数（上） <a class="header-anchor" href="#函数-上" aria-hidden="true">#</a></h1><ul><li>我们代码里面所说的函数和我们上学的时候学习的什么三角函数、二次函数之类的不是一个东西</li></ul><h2 id="函数的概念" tabindex="-1">函数的概念 <a class="header-anchor" href="#函数的概念" aria-hidden="true">#</a></h2><ul><li><p>对于 <code>js</code> 来说，函数就是把任意一段代码放在一个 <strong>盒子</strong> 里面</p></li><li><p>在我想要让这段代码执行的时候，直接执行这个 <strong>盒子</strong> 里面的代码就行</p></li><li><p>先看一段代码</p><div class="language-javascript line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-palenight"><code><span class="line"><span style="color:#676E95;font-style:italic;">// 这个是我们以前写的一段代码</span></span>
<span class="line"><span style="color:#89DDFF;font-style:italic;">for</span><span style="color:#A6ACCD;"> (</span><span style="color:#C792EA;">var</span><span style="color:#A6ACCD;"> i </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">0</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> i </span><span style="color:#89DDFF;">&lt;</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">10</span><span style="color:#89DDFF;">;</span><span style="color:#A6ACCD;"> i</span><span style="color:#89DDFF;">++</span><span style="color:#A6ACCD;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">i</span><span style="color:#F07178;">)</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 函数，这个 {} 就是那个 “盒子”</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fn</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// 这个函数我们以前写的代码</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;font-style:italic;">for</span><span style="color:#F07178;"> (</span><span style="color:#C792EA;">var</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">i</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">=</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">0</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">i</span><span style="color:#F07178;"> </span><span style="color:#89DDFF;">&lt;</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">10</span><span style="color:#89DDFF;">;</span><span style="color:#F07178;"> </span><span style="color:#A6ACCD;">i</span><span style="color:#89DDFF;">++</span><span style="color:#F07178;">) </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">    </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#A6ACCD;">i</span><span style="color:#F07178;">)</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div></li></ul><h2 id="函数的两个阶段-重点" tabindex="-1">函数的两个阶段（重点） <a class="header-anchor" href="#函数的两个阶段-重点" aria-hidden="true">#</a></h2><ul><li>按照我们刚才的说法，两个阶段就是 <strong>放在盒子里面</strong> 和 <strong>让盒子里面的代码执行</strong></li></ul><h3 id="函数定义阶段" tabindex="-1">函数定义阶段 <a class="header-anchor" href="#函数定义阶段" aria-hidden="true">#</a></h3><ul><li><p>定义阶段就是我们把代码 <strong>放在盒子里面</strong></p></li><li><p>我们就要学习怎么 <strong>放进去</strong>，也就是书写一个函数</p></li><li><p>我们有两种定义方式 <strong>声明式</strong> 和 <strong>赋值式</strong></p></li></ul><h4 id="声明式" tabindex="-1">声明式 <a class="header-anchor" href="#声明式" aria-hidden="true">#</a></h4><ul><li><p>使用 <code>function</code> 这个关键字来声明一个函数</p></li><li><p>语法：</p><div class="language-javascript line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fn</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// 一段代码</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// function: 声明函数的关键字，表示接下来是一个函数了</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// fn: 函数的名字，我们自己定义的（遵循变量名的命名规则和命名规范）</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// (): 必须写，是用来放参数的位置（一会我们再聊）</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// {}: 就是我们用来放一段代码的位置（也就是我们刚才说的 “盒子”）</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div></li></ul><h4 id="赋值式" tabindex="-1">赋值式 <a class="header-anchor" href="#赋值式" aria-hidden="true">#</a></h4><ul><li><p>其实就是和我们使用 <code>var</code> 关键字是一个道理了</p></li><li><p>首先使用 <code>var</code> 定义一个变量，把一个函数当作值直接赋值给这个变量就可以了</p></li><li><p>语法：</p><div class="language-javascript line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-palenight"><code><span class="line"><span style="color:#C792EA;">var</span><span style="color:#A6ACCD;"> fn </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// 一段代码</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 不需要在 function 后面书写函数的名字了，因为在前面已经有了</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div></li></ul><h3 id="函数调用阶段" tabindex="-1">函数调用阶段 <a class="header-anchor" href="#函数调用阶段" aria-hidden="true">#</a></h3><ul><li>就是让 <strong>盒子里面</strong> 的代码执行一下</li><li>让函数执行</li><li>两种定义函数的方式不同，但是调用函数的方式都以一样的</li></ul><h4 id="调用一个函数" tabindex="-1">调用一个函数 <a class="header-anchor" href="#调用一个函数" aria-hidden="true">#</a></h4><ul><li><p>函数调用就是直接写 <code>函数名()</code> 就可以了</p><div class="language-javascript line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-palenight"><code><span class="line"><span style="color:#676E95;font-style:italic;">// 声明式函数</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fn</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">我是 fn 函数</span><span style="color:#89DDFF;">&#39;</span><span style="color:#F07178;">)</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 调用函数</span></span>
<span class="line"><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">()</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 赋值式函数</span></span>
<span class="line"><span style="color:#C792EA;">var</span><span style="color:#A6ACCD;"> fn2 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">我是 fn2 函数</span><span style="color:#89DDFF;">&#39;</span><span style="color:#F07178;">)</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 调用函数</span></span>
<span class="line"><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">()</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><ul><li>注意： <strong>定义完一个函数以后，如果没有函数调用，那么写在 {} 里面的代码没有意义，只有调用以后才会执行</strong></li></ul></li></ul><h4 id="调用上的区别" tabindex="-1">调用上的区别 <a class="header-anchor" href="#调用上的区别" aria-hidden="true">#</a></h4><ul><li><p>虽然两种定义方式的调用都是一样的，但是还是有一些区别的</p></li><li><p>声明式函数： 调用可以在 <strong>定义之前或者定义之后</strong></p><div class="language-javascript line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-palenight"><code><span class="line"><span style="color:#676E95;font-style:italic;">// 可以调用</span></span>
<span class="line"><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">()</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 声明式函数</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fn</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">我是 fn 函数</span><span style="color:#89DDFF;">&#39;</span><span style="color:#F07178;">)</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 可以调用</span></span>
<span class="line"><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">()</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div></li><li><p>赋值式函数： 调用只能在 <strong>定义之后</strong></p><div class="language-javascript line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-palenight"><code><span class="line"><span style="color:#676E95;font-style:italic;">// 会报错</span></span>
<span class="line"><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">()</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 赋值式函数</span></span>
<span class="line"><span style="color:#C792EA;">var</span><span style="color:#A6ACCD;"> fn </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">我是 fn 函数</span><span style="color:#89DDFF;">&#39;</span><span style="color:#F07178;">)</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 可以调用</span></span>
<span class="line"><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">()</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div></li></ul><h2 id="函数的参数-重点" tabindex="-1">函数的参数（重点） <a class="header-anchor" href="#函数的参数-重点" aria-hidden="true">#</a></h2><ul><li><p>我们在定义函数和调用函数的时候都出现过 <code>()</code></p></li><li><p>现在我们就来说一下这个 <code>()</code> 的作用</p></li><li><p>就是用来放参数的位置</p></li><li><p>参数分为两种 <strong>行参</strong> 和 <strong>实参</strong></p><div class="language-javascript line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-palenight"><code><span class="line"><span style="color:#676E95;font-style:italic;">// 声明式</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fn</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">行参写在这里</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// 一段代码</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">(实参写在这里)</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 赋值式函数</span></span>
<span class="line"><span style="color:#C792EA;">var</span><span style="color:#A6ACCD;"> fn </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">行参写在这里</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// 一段代码</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">(实参写在这里)</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div></li></ul><h3 id="行参和实参的作用" tabindex="-1">行参和实参的作用 <a class="header-anchor" href="#行参和实参的作用" aria-hidden="true">#</a></h3><ol><li><p>行参</p><ul><li><p>就是在函数内部可以使用的变量，在函数外部不能使用</p></li><li><p>每写一个单词，就相当于在函数内部定义了一个可以使用的变量（遵循变量名的命名规则和命名规范）</p></li><li><p>多个单词之间以 <code>,</code> 分隔</p><div class="language-javascript line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-palenight"><code><span class="line"><span style="color:#676E95;font-style:italic;">// 书写一个参数</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fn</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">num</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// 在函数内部就可以使用 num 这个变量</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#C792EA;">var</span><span style="color:#A6ACCD;"> fn1 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">num</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">	</span><span style="color:#676E95;font-style:italic;">// 在函数内部就可以使用 num 这个变量</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 书写两个参数</span></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fun</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">num1</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#A6ACCD;font-style:italic;">num2</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// 在函数内部就可以使用 num1 和 num2 这两个变量</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#C792EA;">var</span><span style="color:#A6ACCD;"> fun1 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">num1</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#A6ACCD;font-style:italic;">num2</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// 在函数内部就可以使用 num1 和 num2 这两个变量</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br></div></div></li><li><p>如果只有行参的话，那么在函数内部使用的值个变量是没有值的，也就是 <code>undefined</code></p></li><li><p><strong>行参的值是在函数调用的时候由实参决定的</strong></p></li></ul></li><li><p>实参</p><ul><li><p>在函数调用的时候给行参赋值的</p></li><li><p>也就是说，在调用的时候是给一个实际的内容的</p><div class="language-javascript line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fn</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">num</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// 函数内部可以使用 num </span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 这个函数的本次调用，书写的实参是 100</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 那么本次调用的时候函数内部的 num 就是 100</span></span>
<span class="line"><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">100</span><span style="color:#A6ACCD;">) </span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 这个函数的本次调用，书写的实参是 200</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 那么本次调用的时候函数内部的 num 就是 200</span></span>
<span class="line"><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">200</span><span style="color:#A6ACCD;">)</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br></div></div></li><li><p><strong>函数内部的行参的值，由函数调用的时候传递的实参决定</strong></p></li><li><p><strong>多个参数的时候，是按照顺序一一对应的</strong></p><div class="language-javascript line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fn</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">num1</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#A6ACCD;font-style:italic;">num2</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// 函数内部可以使用 num1 和 num2</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 函数本次调用的时候，书写的参数是 100 和 200</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 那么本次调用的时候，函数内部的 num1 就是 100，num2 就是 200</span></span>
<span class="line"><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">100</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">200</span><span style="color:#A6ACCD;">)</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div></li></ul></li></ol><h3 id="参数个数的关系" tabindex="-1">参数个数的关系 <a class="header-anchor" href="#参数个数的关系" aria-hidden="true">#</a></h3><ol><li><p>行参比实参少</p><ul><li><p>因为是按照顺序一一对应的</p></li><li><p>行参少就会拿不到实参给的值，所以在函数内部就没有办法用到这个值</p><div class="language-javascript line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fn</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">num1</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#A6ACCD;font-style:italic;">num2</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// 函数内部可以使用 num1 和 num2</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 本次调用的时候，传递了两个实参，100 200 和 300</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 100 对应了 num1，200 对应了 num2，300 没有对应的变量</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 所以在函数内部就没有办法依靠变量来使用 300 这个值</span></span>
<span class="line"><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">100</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">200</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">300</span><span style="color:#A6ACCD;">)</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div></li></ul></li><li><p>行参比实参多</p><ul><li><p>因为是按照顺序一一对应的</p></li><li><p>所以多出来的行参就是没有值的，就是 <code>undefined</code></p><div class="language-javascript line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fn</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;font-style:italic;">num1</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#A6ACCD;font-style:italic;">num2</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#A6ACCD;font-style:italic;">num3</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// 函数内部可以使用 num1 num2 和 num3</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 本次调用的时候，传递了两个实参，100 和 200</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 就分别对应了 num1 和 num2</span></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 而 num3 没有实参和其对应，那么 num3 的值就是 undefined</span></span>
<span class="line"><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">100</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">200</span><span style="color:#A6ACCD;">)</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div></li></ul></li></ol><h2 id="函数的return-重点" tabindex="-1">函数的return（重点） <a class="header-anchor" href="#函数的return-重点" aria-hidden="true">#</a></h2><ul><li><code>return</code> 返回的意思，其实就是给函数一个 <strong>返回值</strong> 和 <strong>终断函数</strong></li></ul><h3 id="终断函数" tabindex="-1">终断函数 <a class="header-anchor" href="#终断函数" aria-hidden="true">#</a></h3><ul><li><p>当我开始执行函数以后，函数内部的代码就会从上到下的依次执行</p></li><li><p>必须要等到函数内的代码执行完毕</p></li><li><p>而 <code>return</code> 关键字就是可以在函数中间的位置停掉，让后面的代码不在继续执行</p><div class="language-javascript line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fn</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#F78C6C;">1</span><span style="color:#F07178;">)</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#F78C6C;">2</span><span style="color:#F07178;">)</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#F78C6C;">3</span><span style="color:#F07178;">)</span></span>
<span class="line"><span style="color:#F07178;">  </span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// 写了 return 以后，后面的 4 和 5 就不会继续执行了</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;font-style:italic;">return</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#F78C6C;">4</span><span style="color:#F07178;">)</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#F78C6C;">5</span><span style="color:#F07178;">)</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 函数调用</span></span>
<span class="line"><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">()</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br></div></div></li></ul><h3 id="返回值" tabindex="-1">返回值 <a class="header-anchor" href="#返回值" aria-hidden="true">#</a></h3><ul><li><p>函数调用本身也是一个表达式，表达式就应该有一个值出现</p></li><li><p>现在的函数执行完毕之后，是不会有结果出现的</p><div class="language-javascript line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-palenight"><code><span class="line"><span style="color:#676E95;font-style:italic;">// 比如 1 + 2 是一个表达式，那么 这个表达式的结果就是 3</span></span>
<span class="line"><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#A6ACCD;">(</span><span style="color:#F78C6C;">1</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">+</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">2</span><span style="color:#A6ACCD;">) </span><span style="color:#676E95;font-style:italic;">// 3</span></span>
<span class="line"></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fn</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// 执行代码</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// fn() 也是一个表达式，这个表达式就没有结果出现</span></span>
<span class="line"><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#A6ACCD;">(</span><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">()) </span><span style="color:#676E95;font-style:italic;">// undefined</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div></li><li><p><code>return</code> 关键字就是可以给函数执行完毕一个结果</p><div class="language-javascript line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fn</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#89DDFF;">  </span><span style="color:#676E95;font-style:italic;">// 执行代码</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#89DDFF;font-style:italic;">return</span><span style="color:#F07178;"> </span><span style="color:#F78C6C;">100</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#676E95;font-style:italic;">// 此时，fn() 这个表达式执行完毕之后就有结果出现了</span></span>
<span class="line"><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#A6ACCD;">(</span><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">()) </span><span style="color:#676E95;font-style:italic;">// 100</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><ul><li>我们可以在函数内部使用 <code>return</code> 关键把任何内容当作这个函数运行后的结果</li></ul></li></ul><h2 id="函数的优点" tabindex="-1">函数的优点 <a class="header-anchor" href="#函数的优点" aria-hidden="true">#</a></h2><ul><li>函数就是对一段代码的封装，在我们想调用的时候调用</li><li>函数的几个优点 <ol><li>封装代码，使代码更加简洁</li><li>复用，在重复功能的时候直接调用就好</li><li>代码执行时机，随时可以在我们想要执行的时候执行</li></ol></li></ul><h2 id="预解析-重点" tabindex="-1">预解析（重点） <a class="header-anchor" href="#预解析-重点" aria-hidden="true">#</a></h2><ul><li><strong>预解析</strong> 其实就是聊聊 <code>js</code> 代码的编译和执行</li><li><code>js</code> 是一个解释型语言，就是在代码执行之前，先对代码进行通读和解释，然后在执行代码</li><li>也就是说，我们的 <code>js</code> 代码在运行的时候，会经历两个环节 <strong>解释代码</strong> 和 <strong>执行代码</strong></li></ul><h3 id="解释代码" tabindex="-1">解释代码 <a class="header-anchor" href="#解释代码" aria-hidden="true">#</a></h3><ul><li><p>因为是在所有代码执行之前进行解释，所以叫做 <strong>预解析（预解释）</strong></p></li><li><p>需要解释的内容有两个</p><ul><li>声明式函数 <ul><li>在内存中先声明有一个变量名是函数名，并且这个名字代表的内容是一个函数</li></ul></li><li><code>var</code> 关键字 <ul><li>在内存中先声明有一个变量名</li></ul></li></ul></li><li><p>看下面一段代码</p><div class="language-javascript line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-palenight"><code><span class="line"><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">()</span></span>
<span class="line"><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#A6ACCD;">(num)</span></span>
<span class="line"></span>
<span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fn</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">我是 fn 函数</span><span style="color:#89DDFF;">&#39;</span><span style="color:#F07178;">)</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"><span style="color:#C792EA;">var</span><span style="color:#A6ACCD;"> num </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">100</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div></li><li><p>经过预解析之后可以变形为</p><div class="language-javascript line-numbers-mode"><button title="Copy Code" class="copy"></button><span class="lang">javascript</span><pre class="shiki material-palenight"><code><span class="line"><span style="color:#C792EA;">function</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fn</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#F07178;">  </span><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#F07178;">(</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">我是 fn 函数</span><span style="color:#89DDFF;">&#39;</span><span style="color:#F07178;">)</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#C792EA;">var</span><span style="color:#A6ACCD;"> num</span></span>
<span class="line"></span>
<span class="line"><span style="color:#82AAFF;">fn</span><span style="color:#A6ACCD;">()</span></span>
<span class="line"><span style="color:#A6ACCD;">console</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">log</span><span style="color:#A6ACCD;">(num)</span></span>
<span class="line"><span style="color:#A6ACCD;">num </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">100</span></span>
<span class="line"></span></code></pre><div class="line-numbers-wrapper" aria-hidden="true"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div></li><li><p>赋值式函数会按照 <code>var</code> 关键字的规则进行预解析</p></li></ul></div></div></main><!--[--><!--]--><footer class="VPDocFooter" data-v-cfb513e0 data-v-21f75714><!----><div class="prev-next" data-v-21f75714><div class="pager" data-v-21f75714><a class="pager-link prev" href="/docs/frontend/js/%E5%BE%AA%E7%8E%AF%E7%BB%93%E6%9E%84.html" data-v-21f75714><span class="desc" data-v-21f75714>Previous page</span><span class="title" data-v-21f75714>循环结构</span></a></div><div class="has-prev pager" data-v-21f75714><a class="pager-link next" href="/docs/frontend/js/%E5%87%BD%E6%95%B0%EF%BC%88%E4%B8%8B%EF%BC%89.html" data-v-21f75714><span class="desc" data-v-21f75714>Next page</span><span class="title" data-v-21f75714>函数（下）</span></a></div></div></footer><!--[--><!--]--></div></div></div></div></div><footer class="VPFooter has-sidebar" data-v-5211c0b1 data-v-9f24cc86><div class="container" data-v-9f24cc86><p class="message" data-v-9f24cc86>Released under the MIT License.</p><p class="copyright" data-v-9f24cc86>Copyright © 2019-present Evan You</p></div></footer><!--[--><!--]--></div></div>
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